do you know if it is possible to commit less than the actual order qty?

If by commit you are talking about allocation and backorder, then yes you can commit less than the full quantity

how would you do this. like you would do this at the time of fulfullment. basically the order is for 1000 united but we only want to send 500 right now

hmm i assumed it would be a feature/module. but why is there not a native functionality

standard options for Inventory Allocation Preferences are more simple

If you have more available, the system will automatically allocate to that order. When you make the fulfillment, you change the quantity there as well.